An ever-increasing number of relatively inexpensive, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in several IEEE standards documents, including for example, the IEEE Standard 802.11b (1999) and its updates and amendments, as well as the IEEE 802.15.3 Draft Standard (2003) and the IEEE 802.15.3c Draft D0.0 Standard, all of which are collectively incorporated herein fully by reference.
As one example, a type of a wireless network known as a wireless personal area network (WPAN) involves the interconnection of devices that are typically, but not necessarily, physically located closer together than wireless local area networks (WLANs) such as WLANs that conform to the IEEE Standard 802.11a. Recently, the interest and demand for particularly high data rates (e.g., in excess of 1 Gbps) in such networks has significantly increased. One approach to realizing high data rates in a WPAN is to use hundreds of MHz, or even several GHz, of bandwidth. For example, the unlicensed 60 GHz band provides one such possible range of operation.
In general, transmission systems compliant with the IEEE 802 standards support one or both of a Single Carrier (SC) mode of operation or an Orthogonal Frequency Division Multiplexing (OFDM) mode of operation to achieve higher data transmission rates. For example, a simple, low-power handheld device may operate only in the SC mode, a more complex device that supports a longer range of operation may operate only in the OFDM mode, and some dual-mode devices may switch between SC and OFDM modes.
Generally speaking, the use of OFDM divides the overall system bandwidth into a number of frequency sub-bands or channels, with each frequency sub-band being associated with a respective subcarrier upon which data may be modulated. Thus, each frequency sub-band of the OFDM system may be viewed as an independent transmission channel within which to send data, thereby increasing the overall throughput or transmission rate of the communication system. During operation, a transmitter operating in the OFDM mode may encode the information bits (which may include error correction encoding and interleaving), spread the encoded bits using a certain spreading sequence, map the encoded bits to symbols of a 64 quadrature amplitude modulation (QAM) multi-carrier constellation, for example, and transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers, resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR).
Likewise, the receivers generally include a radio frequency (RF) receiving unit that performs correlation and demodulation to recover the transmitted symbols, and these symbols are then processed in a Viterbi decoder to estimate or determine the most likely identity of the transmitted symbol. The recovered and recognized stream of symbols is then decoded, which may include deinterleaving and error correction using any of a number of known error correction techniques, to produce a set of recovered signals corresponding to the original signals transmitted by the transmitter.
Specifically with respect to wideband wireless communication systems that operate in the 60 GHz band, the IEEE 802.15.3c Draft D0.0 Standard (“the Proposed Standard”) proposes that each packet transmitted via a communication channel include a preamble to provide synchronization and training information; a header to provide the basic parameters of the physical layer (PHY) such as length of the payload, modulation and coding method, etc.; and a payload portion. A preamble consistent with the Proposed Standard includes a synchronization field (SYNC) to indicate the beginning of a block of transmitted information for signal detection, a start frame delimiter (SFD) field to signal the beginning of the actual frame, and a channel estimation sequence (CES). These fields can carry information for receiver algorithms related to automatic gain control (AGC) setting, antenna diversity selection or phase array setting, timing acquisition, coarse frequency offset estimation, channel estimation, etc. For each of the SC and OFDM modes of operation, the Proposed Standard specifies a unique PHY preamble structure, i.e., particular lengths of SYNC, SFD, and CES fields as well as spreading sequences and cover codes (sequences of symbols transmitted using the corresponding spreading sequences) for each PHY preamble field.
In addition to being associated with separate structures in SC and OFDM modes, the frame of a PHY preamble consistent with the Proposed Standard fails to address other potential problems such as low sensitivity, for example. In particular, the receiver of a PHY preamble may use either a coherent or a noncoherent method to detect the beginning of the SFD field and accordingly establish frame timing. In general, the coherent method requires channel estimation based on the signal in the SYNC field, which may be performed in an adaptive fashion. However, the SYNC signal may be too short for channel estimation adaptation to converge to a reliable value. On the other hand, the noncoherent method is not based on channel estimation and is generally simpler. However, the noncoherent method is associated with low sensitivity, i.e., frame timing accuracy may be poor at low signal-to-noise (SNR) levels. Because frame timing is critical to receiving the entire packet, low sensitivity in frame timing detection significantly limits overall performance.